HIV infection alters a variety of cellular functions, including cell adhesion and protease secretion. The goals of this project are to determine the functions of integrins and proteases in the response of human monocytes to HIV infection, the mechanisms by which HIV regulates these molecules, their roles in AIDS pathogenesis, and approaches to inhibiting these alterations. Infection of human monocytes by HIV-1 resulted in marked alterations in cell-cell adhesion that could be attributed to changes in levels and function of beta2 integrins. Specifically, infected monocytes aggregated much more with other monocytes (homotypic adhesion) and displayed enhanced adhesion to microvascular endothelial monolayers (heterotypic adhesion). Infected monocytes also displayed increased mRNA levels and secretion of the matrix metalloproteinase MMP-9 (gelatinase B). Endothelial cell monolayers with attached monocytes became disrupted morphologically, and they showed increased permeability to passage of a radiolabeled albumin marker; these effects were inhibited by the metalloproteinase inhibitors TIMP-1 and TIMP-2. We tested the ability of the HIV regulatory molecule Tat alone to affect integrin function and protease induction. Tat treatment mimicked many of the effects of HIV-1 infection. It induced homotypic cell-cell aggregation of treated (but uninfected) monocytes, and it stimulated monocyte adherence to both untreated and TNFalpha-treated microvascular endothelial cell monolayers. The effects could be attributed to changes in the synthesis, surface expression, and function of beta2 integrins. These effects on adhesion could be inhibited by antibodies against the beta2 integrin subunit and its ICAM-1 counter-receptor. Moreover, HIV-Tat treatment alone induced a dramatic induction of MMP-9 synthesis. These studies suggest roles for integrins and proteases in the pathogenesis of AIDS. The changes are consistent with monocyte activation, with enhanced ability to adhere to and disrupt endothelium by means of enhanced integrin and protease expression. We propose that such HIV-activated cells would have an increased propensity to migrate into tissues; such tissue macrophages are known to be major reservoirs of HIV virus. These findings suggest that approaches using specific inhibitors of soluble Tat function or of monocyte activation and invasion might provide new insights into AIDS pathogenesis and preventive therapy.